High-content analysis and isolation of cells is a growth area in personalized medicine. Both white blood cells (WBCs) and circulating tumor cells (CTCs), for example, can provide valuable information for diagnosis and treatment of diseases.
High throughput screening of WBCs can help determine whether a sick patient is responding to a specific drug or a healthy individual has mounted an adequate response to an immunization. Isolated viable WBCs can be used to determine whether specific T cell subpopulations are present in the blood and are capable of eliciting an immune response to the human immunodeficiency virus (HIV).
CTCs, i.e., tumor cells that are identified in transit within the blood stream, are shed from primary and metastatic cancers. Their isolation may be key in understanding the biology of metastasis and in a broad range of clinical applications, including early detection of cancer, the discovery of biomarkers to predict treatment responses and disease progression, as well as monitoring of minimal residual disease following and/or during treatment. Identification of CTC subsets may also allow tailoring of treatment on an individual basis.
Unfortunately, both WBCs and CTCs are rare in whole-blood samples, making their characterization and isolation problematic. Red blood cells (RBCs) typically outnumber WBCs in a whole-blood sample by a ratio of approximately 1000:1. CTCs are extraordinarily rare. An average cancer patient has approximately one to ten CTCs per milliliter of blood (one CTC for every billion blood cells).
Traditionally, gradient separations have been used to separate RBCs from various populations of WBCs. Gradient separations work on the principle that RBCs are small and dense and can form a pellet when whole blood is centrifuged. While effective, the gradient methods are typically slow, difficult to automate, and produce cells with poor viability.
Fluorescence activated cell sorting (FACS) is a well established technique for isolating CBCs from a large population of cells. However, to collect a significant sample of CTCs (e.g., about 10 CTCs) requires the screening of 1010 cells or approximately 2 mL of blood. Ideally the entire analysis should take less than an hour. Thus, the sorter must operate at a throughput of approximately 1 μL/s, corresponding to 5×106 cells/s. This is several orders of magnitude greater than the maximum throughput achievable using FACS. Other automated cell sorting systems are available, but these systems are typically slow, inefficient, expensive, or subject to contamination.
Because WBCs and CTCs typically comprise a very small percentage of the total number of cells in a sample and so are present in very small numbers in a sample, automated processes can be valuable in isolating these rare cells from an adequate sample. Additionally, where a sample is very small or inadequate, such as in fine needle aspirates and samples from embryos or neonates, using automated processes can be equally (if not more) valuable. Therefore, in performing cell analysis, it would be desirable in many applications to have the ability to image, sort, capture, and collect single cells in an automated and high-throughput manner that overcomes the aforementioned and other disadvantages of the prior art.